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Calibrating Exoplanetary Atmospheres Using Benchmark Brown Dwarfs

Periodic Report Summary 1 - CALEXOPLAN (Calibrating Exoplanetary Atmospheres Using Benchmark Brown Dwarfs)

Summary description of the project objectives

Recent advances in Galactic astronomy have largely emerged in the realm of small bodies: the lowest-mass stars, brown dwarfs and planets outside our Solar System. The last has captured the public’s imagination, with thousands of new worlds identified largely through indirect detection techniques (stellar radial velocity variables, transits). Now, the population of directly imaged exoplanets is increasing as major new international planet finding instruments such as the Gemini Planet Finder (GPI) and ESO’s VLT/SPHERE survey nearby young stars for coeval, self-luminous, analogues of Jupiter. Developing tools to characterise these exoplanets in terms of mass, temperature, and composition is crucial for understanding planet formation, and in preparation for the next generation of high contract imaging instruments that will target Neptune analogues in reflected light. This work is reliant on the use of complex model atmospheres for the interpretation of what is often sparse data. As such, robust testing of the model grids against good quality data is crucial. The popular model grids used for studying directly imaged exoplanets have all been developed and first applied to observations of brown dwarfs: free-floating substellar objects in the solar neighbourhood. Some of the greatest outstanding problems in our understanding of both exoplanet and brown dwarf atmospheres centre on the presence of clouds and the role of non-equilibrium chemistry. Clouds are thought to have a dramatic effect on the emergent flux from L type brown dwarfs and directly imaged exoplanets, but their theoretical treatment has significant uncertainties and shortcomings. Non-equilibrium chemistry has been included in some theoretical treatments to provide better matches to data, but its importance and ubiquity is subject to debate.

The CalExoPlan project aims to bring about a step change in our understanding of the problems by robustly calibrating the atmospheric model grids against high quality spectroscopy of benchmark brown dwarfs. To achieve this aim the Fellow, Dr Burningham, has spent two years during the first period of the project on secondment to NASA Ames Research Center (Californian, USA) working with the world leading theorists who developed some of the most popular giant planet and brown dwarf atmosphere codes. The purpose of their collaboration was to train Dr Burningham in the details of atmospheric modelling and thus design new tools for spectroscopic analysis of benchmark systems, model fitting and testing. Dr Burningham will then deploy these tools to interrogate data of known, and newly discovered benchmark brown dwarfs and directly imaged exoplanets, and provide new insights into the cloud properties and chemistry of these exciting objects.

Description of the work performed since the beginning of the project and main results achieved so far

During 2014 - 2016 Dr Burningham worked with Dr Marley and collaborators at NASA Ames Research Center on developing new analysis tools for confronting atmospheric grid models with data. He contributed to groundbreaking retrieval analysis of two cloud-free benchmark T dwarfs (Line et al 2015; ApJ, 807, 183). It became apparent that developing a similar retrieval tool for studying cloudy L-type brown dwarfs and directly imaged planets was the most effective way for achieving the goals of the project - specifically testing model cloud treatments and chemistry. Dr Burningham’s work for the latter 2/3 of the outgoing phase was focused on developing this tool, which reached maturity in the latter third of the outgoing phase. The new tool allows atmospheric thermal profiles, gas abundances and cloud properties to be constrained directly from the data, and compared to model values. A paper describing the new tool and its first use on high signal to noise spectra of L dwarfs has been completed and submitted for publication. The work has thrown up surprising results. Most significantly, the data-driven thermal profiles disagree with popular model grids’ thermal profiles, being cooler at depth and hotter in their stratospheres. Also, the implied cloud location and optical properties are most consistent with iron and/or corundum clouds rather than the silicate clouds that were expected from the model grid predictions. In addition, the work has highlighted a hitherto unforeseen issue with measuring CO abundance from the near-infrared spectra of these objects, a molecule which has already been used to constrain the CO ratio for directly imaged exoplanets. These are the first empirical constraints on either of these properties yet provided for cloudy brown dwarfs or directly imaged exoplanets.

During this time, Dr Burningham joined the Gemini Planet Imager Exoplanet Survey, and has been contributing to their observations searching for new exoplanets, and the analysis of their discoveries, including the discovery of a jovian mass planet orbiting the 30 Myr old star 51 Eri (Macintosh et al 2015, Science, 350, 64).

Dr Burningham has continued to work in collaboration with researchers at the University of Hertfordshire to identify new benchmark brown dwarfs, and retrieval analysis of these targets is planned for the future.

The expected final results and their potential impact and use

During the final phase of the project Dr Burningham is focusing on applying the new analysis tool to a variety of benchmark brown dwarf systems to further validate his approach and provide new insights to their atmospheric conditions to improve the model grids. He has also started to apply the tool to the analysis of directly imaged exoplanets, which are currently very poorly matched by the same model grids. By applying techniques developed and calibrated as part of the CalExoPlan project he will be able to robustly measure the composition of directly imaged giant exoplanets, providing crucial insights into their formation.